Conventionally, there has been provided a known structure of, when driving a camshaft by using a timing pulley and a chain sprocket which rotate in synchronization with an engine crankshaft, rotating the camshaft relatively to the crankshaft by using a variable valve timing device of vane type disposed between the timing pulley and the camshaft so as to retard or advance the angle of the rotation of the camshaft with respect to the rotation of the crankshaft to shift the operating timing of an intake valve and that of an exhaust valve with respect to the rotation of the engine, thereby reducing the exhaust gas and providing an improvement in the fuel consumption.
In addition, as a solenoid valve for controlling the above-mentioned variable valve timing device, there has been provided a solenoid valve described in, for example, patent reference 1. An example of this type of solenoid valve is shown in FIG. 8. This solenoid valve 30 consists of a spool 41 which is driven by a solenoid unit 50, and a valve housing 40 for accommodating the spool 41 therein in such a way that the spool 41 can slide in a direction of the axis thereof, in which an oil passage is formed. In this valve housing 40, an oil supply port 45 which is opened and closed by the spool 41 and which communicates with an oil supply source, and advance angle side and retard angle side ports 43 and 44 which can communicate with the variable valve timing device are disposed. In addition, communicating paths 90 which can communicate between the oil supply port 45 and the advance angle side and retard angle side ports 43 and 44 respectively are formed.
When performing an operation of intermediately holding the variable valve timing device, the solenoid valve 30 controls the variable valve timing device by blocking the hydraulic pressure supply to the variable valve timing device to supply an oil flow having a very low amount of flow to the variable valve timing device. At that time, an oil leakage from the oil passage and so on occurs, and therefore the stability of the intermediately holding operation degrades. To solve this problem, in the solenoid valve disclosed in patent reference 1, the communicating path 90 is disposed to supply an amount of oil which can compensate with the amount of oil leaking from the oil passage and so on so as to ensure the stability in the operation of intermediately holding the variable valve timing control device.    [Patent reference 1] JP,2003-214552,A
Because the conventional solenoid valve for variable valve timing control device is constructed as mentioned above, the conventional solenoid valve can increase the amount of oil supply via the communicating path in the operation of intermediately holding the variable valve timing device while the solenoid valve supplies oil having a large amount of flow to the oil supply port when, for example, operating the variable valve timing control device on an advance angle side. A problem with the conventional solenoid valve is that at that time, the oil is supplied through the communicating path because the advance angle side port serves as a supply of the oil while because the retard angle side port serves as an outlet of the oil, there occurs a state in which a part of the oil fed via the oil supply port is always discharged via the communicating path, and the amount of oil leakage increases in the whole solenoid valve.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a solenoid valve for variable valve timing control devices and a variable valve timing control system that can prevent the amount of oil leakage from increasing in the whole solenoid valve when operating the variable valve timing control device on an advance angle or retard angle side and that can ensure an adequate amount of oil supply when operating the variable valve timing control device in an intermediate holding state.